Hello, i saw the theory behind the differential amplifier and now im building one.

VCC is 12V, VEE = -12V
Itail = 2mA, so ic on each resistor needs to be 1mA.

What i did is first of all calculating the needed resistor in order to have 2mA on R1, so its -VEE-(-VBE)/R1=2*10^(-3) , that does R1=5.65k

now i know that each ic needs to be 1mA and vc needs to be at half of vcc, in order to have maximum swing, so vc is at 6V
so R2=R3 = 6k, is that correct to proceed in this way? what signals i can apply to V3 and V3 before entering in saturation? ( its 6V sine wave or 12V sine wave, similar to vcc value (?).

I got some problems also with my simulation, since when i click on sine generators i got this: ( what u can see in the imagine) how do i fix that? can someone help?

Thanks to everyone.

 

hi 089,
Why have you chosen these setting V3 source settings?
100u ?????
E

 

hi 089,
Why have you chosen these setting V3 source settings?
100u ?????
E
View attachment 349496

ah lmao, my bad...

 

hi,
This is plot using 10kHz for V3.
BTW: please assign labels to your test points, it makes it easier to read the plots.
E

 

hi 089,
Why have you chosen these setting V3 source settings?
100u ?????
E
View attachment 349496

ok that is fixed now, thanks

 

hi,
This is plot using 10kHz for V3.
BTW: please assign labels to your test points, it makes it easier to read the plots.
E
View attachment 349497

now how VD1 and VD2 gonna work? on there i will get the difference beetwen V3 and V4 signals no?, in my case since they are both 2V, if i put myself on vd1-vd2 i will see 0V, but what happens if VD1 is like at 3v? ( what i expect is 3-1, so 2V sine wave? ) another thing that i dont understand is why in this situation if i click on "VD1" node what i see is a sine wave that goes from 6 till down until 5V and the as high as 7V

 

hi,
Look at and run this simulation.
E

 

hi,
Look at and run this simulation.
EView attachment 349499

why the sine wave doesnt have the same phase? also why in my circuit it didnt work like that? and how do i understand whats the max va and vb i can apply?

 

why the sine wave doesnt have the same phase?

hi,
It is a 'Push Pull' amplifier, so the inputs are in antiphase.
Note: you could connect one input to 0V, via a resistor. try that option.

how do i understand whats the max va and vb i can apply?

It depends upon the Gain, you must avoid saturating the Voutput swing.

What do you calculate the Gain to be of the circuit???


E

 

hi,
It is a 'Push Pull' amplifier, so the inputs are in antiphase.
Note: you could connect one input to 0V, via a resistor. try that option.



It depends upon the Gain, you must avoid saturating the Voutput swing.

What do you calculate the Gain to be of the circuit???


E

well in this case i have no idea how i can set the gain of the circuit , cus i saw that the relation RC/RE doesnt work properly , so like 6k/5.65k = 1.065, but i saw the gain is much higher

I would like to have a gain of 1 for example, but keeping same parameters, so ie=2mA and ic=1mA

 

hi 089,
Check this Design PDF.
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hi 089,
Check this Design PDF.
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Yeah i know that with such design im gonna solve my gain issue, since the current generator has a high internal resistor ( ideally ).

But i didnt get yet how to calculate the gain that i get on each signal ( so on VD1 and VD2 )

beside the design itself, in order to have numbers

 

hi,
The PDF I posted clearly shows how to calculate the Gain.

Is this a Homework or College assignment??

E

 

hi,
The PDF I posted clearly shows how to calculate the Gain.

Is this a Homework or College assignment??

E
View attachment 349507

nope, as i told u i dont go to college anymore, right now i work and try study things by myself

 

hi,
The PDF I posted clearly shows how to calculate the Gain.

Is this a Homework or College assignment??

E
View attachment 349507

Yeah, i tried applying this formula to my circuit, but apparently its not true Ad = - RC/RE+ re.

Now i set a RC of 6kOhm and RE is 5.65k so the gain would be around 1,06, but if u apply a signal at va of 1v for example, and extract it on VC u wont see 1,06V , but a lot higher signal, u can check by urself by putting values in the circuit u sent me, thanks.

 

i guess i understood, its the dc component given by VC, now im trying to remove it by using 2 caps on each VC node, but it doesnt work.

 

Hi 089,
Please post your LTS asc files, it helps me to see your results.
E

 

Hi 089,
Please post your LTS asc files, it helps me to see your results.
E

sure here u go

 

Hi,
The DC is removed, the Vout swings above and below zero volts OK.
E

 

Hi,
The DC is removed, the Vout swings above and below zero volts OK.
E
View attachment 349513

YEE, i saw also, i added a load of 100k to gnd and it worked out D